Electrical transformers use inductive coupling of windings, in combination with a core of magnetic material, to transfer power from one circuit to another while maintaining galvanic isolation between the two circuits, in addition to increasing or decreasing output AC voltage levels. The galvanic isolation is needed to power devices that come into contact with people to provide safety and avoid electrocution. Typical transformers use insulating materials such as winding insulation or plastic bobbins between the windings and core to prevent them from touching and maintain galvanic isolation. In some commercial devices, particularly medical equipment, the danger of electrocution of patient or medical staff is so great that additional steps are taken to ensure galvanic isolation and prevent electrical breakdown even in the presence of large voltages as high as 5 kV that may appear, for example, in defibrillators. Typically an isolation barrier of nonconductive material is inserted in the middle of the transformer. Input and output windings are positioned on opposite sides of the barrier and a possibly undesired, but unavoidable air gap is incorporated in the magnetic core and thus the path of the magnetic flux. This barrier technique allows a transformer to be constructed as two separable halves so power source and the powered circuit may be conveniently separated and mated back together again. This technique has been used in rechargeable patient monitoring equipment, and in rechargeable toothbrushes and shavers where an alternative technique, such as the use of exposed electrical contacts, is particularly undesirable because of the moist environment causing corrosion, shorting and increased shock hazard.
In traditional transformer design, the magnetically permeable core containing the magnetic flux, is designed to have an approximately equal cross section (taken perpendicular to the direction of the magnetic flux) throughout the core length to make the most efficient use of the core material. In addition, an air gap is sometimes used to increase the magnetic reluctance in order to allow a higher level of ampereturns in the windings before core saturation occurs, to reduce core losses, or to linearize the B-H curves for the transformer for use in applications such as filters where distortion is of concern, for example. Known barrier technique transformers continued the traditional practice of utilizing cores of approximately constant cross section although this is not the optimum design where the magnetic circuit includes a large air gap.
Known barrier technique transformer devices are typically relatively inefficient, well below the 80% level achievable in conventional DC-DC converters. This wastes power and creates excess heat. Wasted power is important in power constrained applications such as a standard computer port or interface such as USB (Universal Synchronous Bus), for example. In addition, the power density of known barrier technique transformer devices is typically relatively low, restricting its use to low power devices such as toothbrushes and shavers (where recharging power may be 1 to 3 watts, for example), or alternatively such devices involve relatively large and heavy transformer magnetic components. These disadvantages result from poor coupling between input and output windings.
Transformer based power coupling with barrier isolation is used in many portable device type applications including in portable patient monitoring systems. Such portable patient monitoring systems also involve maintaining a data link between the monitoring equipment and a central location, while a portable device is in transit with a patient. Power to the monitoring equipment during transit is typically provided by batteries in the monitoring equipment. One skilled in the art will understand that batteries require charging, and that patients are in transit a small fraction of the time. Current portable monitoring equipment includes fixed docking stations in all appropriate fixed locations, such as operating rooms, examining rooms and patient rooms. When a patient is in one of these locations, the portable monitoring equipment is inserted into the docking station at that location. These docking stations are connected to the AC power at that location, and provide charging current for the batteries in the monitoring equipment. This permits the batteries to maintain their charge. When a patient is moved, the monitoring equipment, with a charged battery, is removed from the docking station, and transported with the patient until another docking station is available.
When the monitoring equipment is docked a wireless data link, e.g. radio frequency (RF), link typically transmits monitoring data from the monitoring equipment to the central location. Each piece of monitoring equipment includes an RF transceiver and antenna. Each docking station also includes a corresponding RF transceiver and antenna. In addition, free-standing antennas and transceivers are located throughout the hospital, in particular at locations where patients would be transported, e.g. halls, etc. Each of the transceivers in the docking stations and the free standing locations is connected by a wired connection to the central location. Using RF communications between the docking station and the monitoring equipment further provides electrical isolation.
When a patient is in a fixed location, and the monitoring equipment is placed in a docking station, the docking station receives the RF signal from the monitoring equipment and transmits the data to the central location via its wired connection. When a patient is in transit from one fixed location to another, the free standing antennas/transceiver locations receive the RF signal from the monitoring equipment and transmit the data to the central location. This provides the ability to monitor a patient continuously. However, there are locations in which continuous RF transmissions from the monitoring equipment may cause problems and must be carefully planned for. For example, in operating rooms, electro-cautery machines use RF energy to cut tissue and coagulate blood during surgery. This instrument causes an unpredictable amount of RF energy and could possibly interfere with the RF link of the monitoring equipment. However, it is in this environment that it is most important that no monitoring data be lost or corrupted.
A system according to the principles of the invention addresses the power coupling and data link problems previously discussed as well as derivative problems.